1. Field of the Invention
The present invention relates to forming spring elements in three dimensions, and more particularly, to a method and system for batch forming spring elements in three dimensions using a configurable die.
2. Background of the Invention
Electrical interconnects or connectors are used to connect two or more electronic components together or to connect an electronic component to a piece of electrical equipment, such as a computer, router, or tester. The term “electronic component” includes, but is not limited to, printed circuit boards, and the connector can be a board-to-board connector. For instance, an electrical interconnect is used to connect an electronic component, such as an integrated circuit (an IC or a chip), to a printed circuit board. An electrical interconnect is also used during integrated circuit manufacturing for connecting an IC device under test to a test system. In some applications, the electrical interconnect or connector provides a separable or embedded or remountable connection so that the electronic component attached thereto can be removed and reattached. For example, it may be desirable to mount a packaged microprocessor chip to a personal computer motherboard using a separable interconnect device so that malfunctioning chips can be readily removed, or upgraded chips can be readily installed.
There are also applications where an electrical connector is used to make direct electrical connection to metal pads formed on a silicon wafer. Such an electrical connector is often referred to as a “probe” or “probe card” and is typically used during the testing of the wafer during the manufacturing process. The probe card, typically mounted on a tester, provides electrical connection from the tester to the silicon wafer so that individual integrated circuits formed on the wafer can be tested for functionality and compliance with specific parametric limits.
Conventional electrical connectors are usually made of stamped metal springs, which are formed and then individually inserted into an insulating carrier to form an array of electrical connection elements. Other approaches to making electrical connectors include using isotropically conductive adhesives, injection molded conductive adhesives, bundled wire conductive elements, springs formed by wirebonding techniques, and small solid pieces of metal.
Land grid array (LGA) refers to an array of metal pads (also called lands) that are used as the electrical contact points for an integrated circuit package, a printed circuit board, or other electronic component. The metal pads are usually formed using thin film deposition techniques and are coated with gold to provide a non-oxidizing surface. Ball grid array (BGA) refers to an array of solder balls or solder bumps that are used as the electrical contact points for an integrated circuit package. Both LGA and BGA packages are widely used in the semiconductor industry and each has its associated advantages or disadvantages. An LGA connector is usually used to provide removable and remountable socketing capability for LGA packages connected to PC boards or to chip modules.
Advances in electronic device packaging technology have led to shrinking package geometries and increasing lead count. That is, the spacing (or the pitch) between each component electrical connection (also referred to as a “lead”) on an electronic device is decreasing, while the total number of connections is increasing. For example, existing IC packages may be built with a pitch of one mm or less with 600 or more connections. Furthermore, IC devices are designed to be operated at increasingly higher frequencies. For example, IC devices for use in telecommunication and networking applications can include input and output signals at frequencies over 1 GHz. The operating frequencies of the electronic devices, the package size, and lead count of the device packages place stringent requirements on the interconnect systems used to test or connect these electronic devices.
Advances in semiconductor technologies have also led to shrinking dimensions within semiconductor integrated circuits, and particularly to decreasing pitch for the contact points on a silicon die or a semiconductor package. For example, contact pads on a semiconductor wafer can have a pitch of 250 microns or less. At the 250 micron pitch level, it is prohibitively difficult and expensive to use conventional techniques to make separable electrical connections to these semiconductor devices. The problem is becoming even more critical as the pitch of contact pads on a semiconductor device decreases below 50 microns and simultaneous connection to multiple contact pads in an array is required. In particular, the mechanical, electrical, and reliability performance criteria of an interconnect system are becoming increasingly demanding. Conventional interconnect technologies have not been able to meet all of the mechanical, electrical, and reliability requirements for use with high speed, small dimension, and large pin count IC devices.
A particular problem encountered by today's interconnect systems is the variation in coplanarity (vertical offset) and positional misalignment of the leads in the electronic components to be connected. Coplanarity variations result in some contact elements being compressed more than others. This difference results primarily from the sum of the following three factors: (1) variations in the planarity of the package, (2) variations in the planarity of the board, and (3) any tilting of the package with respect to the board.
In a conventional LGA package, the pads (the leads) of the package can become non-planar due to substrate warping. When the amount of the resulting vertical offset exceeds the tolerance of a LGA connector, some of the pads may not be able to make electrical contact with the connector at all. Planarity variations of the pads of an LGA component make it difficult to make high quality and reliable electrical connections to all the leads of the electronic component.
Moreover, the location of the leads may also deviate from their predefined ideal position due to manufacturing limitations, resulting in positional misalignment. An effective interconnect must accommodate the horizontal positional variations of the leads of the electronic components to be connected. To make matters worse, the positional deviation of a lead relative to the lead size itself, due to either coplanarity variations, positional misalignments, or both, on an electronic device from its ideal location increases as the size of the package decreases.
Planarity problems are not limited to IC packages but may also exist on the printed circuit board (PCB) to which these IC packages are attached. Planarity problems may exist for LGA pads formed as an area array on a PCB due to warping of the PCB substrate. Typically, deviation from flatness in a conventional PCB is on the order of 50 to 75 microns or more per inch. The LGA connector must be able to accommodate the overall deviations in coplanarity between the components being connected, a package and a PCB for example. This means that the contact elements must function in both the least compressed state, where the curvature and tilt of the package and PCB are such that they are farthest apart from each other, and the most compressed state, where the curvature and tilt of the package and PCB are such that they are closest together. Hence, it is desirable to have a scalable electrical contact element that can behave elastically so that normal variations in coplanarity and positional misalignment of the contact points can be tolerated.
While LGA connectors can be effectively used to electrically connect an LGA package to printed circuit boards or modules, the connector interface between the connector and the component to be connected are subject to potential reliability degradation. For instance, corrosive materials or particulate debris can enter the interface area, preventing a proper electrical connection from being made. Also, the repeated mating and separation of an LGA package may degrade the LGA connector, causing intermittent connection conditions and inhibit reliable electrical connection.
When making electrical connections to contact pads, such as metal pads on a silicon wafer or on a LGA package, it is important to have a wiping action or a piercing action when the contact elements engage the pads in order to break through any oxide, organic material, or other films that may be present on the surface of the metal pads and that might otherwise inhibit the electrical connection. FIG. 1 illustrates an existing contact element engaging a metal pad on a substrate. Referring to FIG. 1, a connector 100 includes a contact element 102 for making an electrical connection to a metal pad 104 on a substrate 106. The connector 100 can be a wafer probe card and the contact element 102 is then a probe tip for engaging the pad 104. Under normal processing and storage conditions, a film 108, which can be an oxide film or an organic film, forms on the surface of the pad 104. When the contact element 102 engages the pad 104, the contact element 102 must pierce through the film 108 in order to make a reliable electrical connection to the pad 104. The piercing of the film 108 can be performed by a wiping action or a piercing action of contact element 102 when the contact element 102 engages the pad 104.
While it is necessary to provide a wiping or piercing action, it is important to have a well-controlled wiping or piercing action that is strong enough to penetrate the surface film 108 but soft enough to avoid damaging the metal pad 104 when electrical contact is made. Furthermore, it is important that any wiping action provides a sufficient wiping distance so that enough of the metal surface is exposed for a satisfactory electrical connection.
Similarly, when making contacts to solder balls, it is important to provide a wiping or piercing action to break through the native oxide layer on the solder balls to create a good electrical contact to the solder balls. However, when conventional approaches are used to make electrical contact to solder balls, the solder balls may be damaged or dislodged from the package. FIG. 2a illustrates the existing contact element 100 being applied to contact a solder ball 200 formed on a substrate 202. When the contact element 102 contacts the solder ball 200, such as for testing, the contact element 102 applies a piercing action which often results in the formation of a crater 204 on the top surface (also called the base surface) of the solder ball 200.
When the substrate 202 is subsequently attached to another semiconductor device, the crater 204 in the solder ball 200 can lead to void formation at the solder ball interface. FIGS. 2b and 2c illustrate the result of attaching the solder ball 200 to a metal pad 210 of a substrate 212. After solder reflow (FIG. 2c), the solder ball 200 is attached to the metal pad 210. However, a void 214 is formed at the solder ball interface due to the presence of the crater 204 on the top surface of the solder ball 200. The presence of the void 214 can affect the electrical characteristics of the connection and more importantly, degrades the reliability of the connection.
Conventional interconnect devices, such as stamped metal springs, bundled wire, and injection molded conductive adhesives, become difficult to manufacture as the dimensions are scaled down. Stamped metal spring elements, in particular, become brittle and difficult to manufacture as the dimensions are scaled down, rendering them unsuitable for accommodating electronic components with normal positional variations. This is particularly true when the spacing between the contacts scales below one millimeter, as well as where the electrical path length requirement also scales to below one millimeter to minimize inductance and meet high frequency performance requirements. At this size, spring elements made by existing manufacturing technologies become even more brittle and less elastic and cannot accommodate normal variations in system coplanarity and positional misalignments with a reasonable insertion force of about 30 to 40 grams per contact.
It is desirable to provide an electrical contact element that can provide a controlled wiping action on a metal pad, particularly for pads with a pitch of less than 50 microns. It is also desirable that the wiping action provides a wiping distance of up to 50% of the contact pad. Furthermore, when electrical contact to solder balls are made, it is desirable to have an electrical contact element that can provide a controlled wiping action on the solder ball without damaging the contact surface of the solder ball.
It is desirable to provide an electrical interconnect system which can accommodate normal positional tolerances, such as coplanarity variations and positional misalignments, in electronic components to be connected. Furthermore, it is desirable to provide an electrical interconnect system adapted for use with small geometry, high lead density electronic devices operating at high frequencies.
Existing methods and systems of forming spring elements in three dimensions have utilized custom tools, which are often designed for a specific size spring element, are not configurable, and are expensive to manufacture. There is therefore a need for a method and system for forming spring elements in three dimensions that is flexible, configurable to different spring element characteristics, and low in cost.